The present invention is directed to a method for evaluating logs to predict warping tendency of lumber that might be cut from a given log. The method can be used in a sorting yard, on-line in a sawmill, or at other locations along the route from forest to mill. It enables decisions whether a log should be directed to a sawmill for lumber manufacture or for other applications such as timbers, plywood, or pulp chips. The method enables a sawyer to make real time decisions as to how a log should be cut to obtain maximum product utility.
The majority of the available old growth softwood forests in the world have now been harvested. This wood has now been replaced in many areas of the world by trees grown on intensively managed plantations or xe2x80x9ctree farmsxe2x80x9d. Over the years nurseries producing seed for plantation trees have used intensive genetic selection to improve such heritable traits as rapid growth, straightness of stem, reduced limb diameter, and other desirable characteristics. Most growth cycles now include one or more fertilizations. Plantation trees are also usually thinned and may be pruned one or more times. While plantations now provide a dependable supply of wood for lumber and pulp, the transition from old growth to plantation wood has seen a significant change in size and characteristics of the wood supplied to the mills. Depending on the species and growth locale, plantation trees for saw logs are usually harvested on a 20-50 year growth cycle. The various pine species are usually harvested 20-30 years after planting and typically produce logs having a butt diameter about 30-60 cm in diameter.
It is the nature of most conifer species to produce wood having so-called juvenile characteristics during the first 10-15 years of their growth. This juvenile wood is characterized by thinner cell (tracheid) walls and a higher microfibril angle in the tracheid walls. One characteristic of juvenile wood is reduced density. Another, attributed to the greater microfibril angle, is greater longitudinal shrinkage on drying. Density increases as wood is laid down at greater distances from the pith and the microfibril angle decreases until wood laid down after about 12-15 years growth has acquired xe2x80x9cadultxe2x80x9d properties. Under normal conditions, density and microfibril angle then remain essentially constant during the remaining years of the tree""s growth. This difference in properties radially across the logs can affect the strength and other properties of lumber sawn from the trees. Density is known to correlate directly with modulus of elasticity. Further, the difference in longitudinal shrinkage from pith to outer wood can be responsible for warp of lumber produced from the logs, particularly the defects known as bow and crook.
Various means have been proposed to overcome the above problems. For example, U.S. Pat. No. 6,001,452 to Bassett et al. shows a composite lumber product in which the denser wood from the outer portions of the tree is selectively located in a composite lumber product to improve bending strength. Similarly, published PCT application WO 00/12230 to Stanish et al. describes a method of predicting warp potential by estimating lengthwise shrinkage rates and measuring grain angle of lumber.
Snyder et al., in U.S. Pat. No. 6,026,689, describe a method of estimating modulus of elasticity of wood (MOE) in a log by impacting the log with a pneumatic hammer and measuring velocity of the resulting stress wave. Related technology is described in PCT Applications WO 00/11467 and WO 01/09603 and British Patent 1,244,699. In general, low stress wave velocity correlates with lower modulus wood. Technology of this general type is used with the present invention.
The method described by Stanish et al. is more applicable to cut lumber than to raw logs. It would be extremely useful if warp propensity could be reliably determined earlier in the process, such as in the sorting yard or even with standing trees, so that logs that would produce warp prone lumber might be directed to other products. Examples could be solid sawn timbers or veneers for plywood where potential dimensional instability poses much less of a problem. Alternatively, other uses such as particle, flake, or oriented strand boards or chips for wood pulp would be possible.
In a study of 75 small pine logs, F. G. Wagner and F. W. Taylor suggest a possible relationship between log sweep with bow or crook of finished lumber (Impact of log sweep on warp in southern pine structural lumber, Forest Products Journal, 45(2): 59-62 (1995)).
The present invention is directed to a procedure that makes a highly predictable early warp propensity evaluation of raw logs both possible and fully compatible with operations in the forest, at a sort yard, or in an integrated sawmill.
The present invention is a method that enables prediction, before the log is milled, of crook and bow that might occur in lumber sawn from a given log. The method includes determination of stress wave velocity in the log by known methods. The method further includes determination of the external geometric configuration of the log which may be done by known scanning methods. Data from these two determinations are then included in a multivariate regression equation that can predict with considerable accuracy the warp tendency of lumber that might be milled from the log. The word xe2x80x9clogxe2x80x9d should be read to include the stem of standing trees, felled and limbed trees, and felled trees cut into appropriate lengths for processing in a sawmill. In most environments the necessary stress wave and geometric data can be determined on a real time basis before the log is sawn. Stress wave velocity can be programmed into the existing computers associated with log geometry scanners to determine what type of product should be produced from the log. If a high tendency for warp is indicated, the log might be sawn into timbers rather than dimension lumber. Alternatively, it could be directed to production of plywood, wood composites, or chips for pulp manufacture.
Log scanners are available from a number of manufacturers. These are in common use in sawmills and plywood mills. Among other benefits, they can determine the best orientation of the log as it is presented to the primary breakdown saws. Scanners also typically determine optimum settings for primary breakdown and secondary processing saws in order to obtain maximum product value. In the case of plywood, scanners determine the optimum centers for chucking the lathe block.
While there are differences in method of operation and data determined, most log scanners will make a multiplicity of circumferential scans along the length of the log to determine such properties as large and small end diameters, cross sectional shape, and sweep. Sweep is a departure from longitudinal axis linearity in one or more planes. These scans may determine location of a hundred or more points at each of successive log circumferences to determine cross sectional configurations and position them orthogonal to a longitudinal reference line generally parallel to the log being measured. The circumferential configurations are indicative of the log cross section and their displacement from the reference line at the scan location. Successive scans may be from about 1-30 cm apart. These geometric measurements may be digitized and define the outer configuration of the log. The resulting data can be readily manipulated by an associated computer to automatically program downstream manufacturing parameters.
Stress wave velocity analysis is also now frequently used for estimation of thestiffness or modulus of elasticity of logs before lumber manufacture. Various types of apparatus are now commercially available for this measurement. Most will use one or more sensors contacting the log to measure the time of travel of a sound wave produced by a hammer or other device impacting the log. Stress wave time may be determined over the full length of the log or some incremental portion. It can be measured on felled logs or standing trees.
Stress wave velocity has been used in an attempt to predict warp tendency of lumber. However, the correlation has generally been so poor as to be of minimum value. The present inventors have now discovered that stress wave velocity in combination with certain log geometric parameters can be used to predict lumber warp with considerable accuracy. The geometric parameters can include but are not limited to such log defects as sweep or cross section irregularity. Using a sample population of logs, a multivariate regression equation must first be determined to set up the relationship for each species and general geographic area where the trees are grown. A representative group of trees, generally at least about 10 and preferably 50 or more, are examined for stress wave velocity and geometric configuration. These trees are then sawn conventionally except that the lumber from each tree is marked so that its source tree is known. After drying and subsequent finishing operations any observed warp of the lumber is measured. This warp can then be related to the earlier measured properties of the tree.
Inclusion of stress wave velocity with even a single geometric parameter in the regression equation has almost doubled the predictability accuracy compared with stress wave velocity standing alone. For prediction of crook, one such geometric parameter is the average difference in center locations of adjacent cross sections relative to the longitudinal reference line. This is a measurement indicative of log sweep. Inclusion of additional geometric measurements further increases the predictability. Similarly, measurements related to log eccentricity or other cross sectional irregularity when combined with stress wave velocity, are predictive of bow in the sawn lumber.
It is an object of the present invention to provide a method that has reliable predictive power indicating which logs might produce warp prone lumber.
It is a further object to provide a method that uses stress wave velocity and log geometric parameters in combination to indicate logs that might produce warp-prone lumber.
It is another object to create a multivariate regression equation with log stress wave velocity and log geometric parameters as independent variables predictive of lumber warp.
It is yet an object to use log geometric parameters indicating sweep and cross sectional eccentricity in combination with stress wave velocity as lumber warp predictors.
These and many other objects will become readily apparent to those skilled in the art upon reading the following detailed description taken in conjunction with the drawings.